Electric actuating devices, such as rotating or linear moving electric apparatus, dynamos, motors, generators, etc., typically include a moving commutator. The commutator is electrically coupled to an external electric circuit through one or more brushes which make physical contact with the commutator. As the commutator moves against the brush, the contact surface of the brush wears down to a point where replacement of the brush is required.
Brush wear detectors are known in the art and generally comprise various types of mechanical and electrical arrangements which act to signal the fact that the brush has worn away to a point at which replacement is required. Known detectors may comprise electrical leads inserted into the brush which signal that the brush is worn. An example of such an apparatus is shown and described in U.S. Pat. No. 5,870,026, entitled “Brush Wear Indicator,” which issued to Keith C. Challenger on Feb. 9, 1999. Detectors that utilize electrical leads inserted into the brush not only increase the cost and complexity of the detector system, but may also cause metal-on-metal structural damage if the brushes are not replaced and the leads contact the commutator for an extended period of time.
Another example of a brush wear detector is one in which a magnet moves towards the commutator as the brush wears down and activates a reed switch at a point when the brush needs to be replaced. An example of such an apparatus is shown and described in U.S. Pat. No. 4,739,208, entitled “Brush Assembly Including Brush Wear Detector,” which issued to Dan W. Kimberlin on Apr. 19, 1988. Reed switches, however, are mechanical devices and are susceptible to shock and vibration which may be encountered in electric actuating devices.
Other examples of brush wear detectors are those which depend on physical contact between metallic components of the brush assembly to complete an electrical circuit. Examples of such an apparatus are shown and described in: U.S. Pat. No. 6,255,955, entitled “Brush Warning Indicator and Methods For Indicating Brush Wear-Out,” which issued to Harald Edmund Blaettner on Jul. 3, 2001; U.S. Pat. No. 5,731,650, entitled “Dynamoelectric Machine With Brush Wear Sensor,” which issued to Walfried F. Scheucher on Mar. 24, 1998; and U.S. Pat. No. 4,950,933, entitled “Carbon Brush Holder utilizing a Worn Brush Detector,” which issued to James R. Pipkin et al. on Aug. 21, 1990. Such detectors are not only costly and complicated, but are susceptible to unreliability if the contact parts become corroded or are fouled by foreign particulates such as dust from worn brushes.
Therefore, it is an object of the present invention to provide an improved brush wear detector that does not depend on physical contact between metallic components of the brush assembly.